The present invention relates to a system method (and apparatus) for locating and measuring an index mark on an edge of a wafer, and more particularly, to a method of locating an index mark with the wafer positioned in a horizontal position and of measuring the index mark with the wafer in a vertical position.
An index mark, such as, for example, a flat or a notch is provided on the edge of a semiconductor wafer by the wafer manufacturer so as to permit reliable and repeatable positioning of wafers in automated semiconductor processing facilities. Typically, one surface and an edge of a wafer, including the index mark, are highly polished. Various methods of finding an index mark along the edge of a wafer are practiced or have been proposed. Such methods include mechanical or opto-mechanical index mark sensing in production equipment, as well as locating an index mark by optical mapping of a wafer edge during rotation of the wafer as, for example, proposed by Cheng in U.S. Pat. No. 5,546,179. The method disclosed by Cheng requires the mapping of the entire edge or periphery of a wafer in order to locate an index mark by rotating the wafer fully 360xc2x0 while the wafer surface near the edge of the wafer is interrogated by a focussed beam of electromagnetic radiation. While the method proposed by Cheng is capable of determining the height of a wafer as well as the reflectivity of the wafer surface, this method appears to preclude the measurement of an index mark in which the beam emitted by the sensor device would be directed and focussed at the index mark across a thickness dimension of the wafer.
In order to enhance the throughput of wafers in a wafer testing facility, it is desirable to minimize the time required for locating an index mark, and it is considered important to measure directly the optical properties of an index mark so as to ascertain which wafers to accept or to reject on the basis of a degree of optical reflectivity and the uniformity thereof of an index mark.
It is an object of the present invention to provide a system for locating an index mark on the edge of a wafer with the wafer in a horizontal position, and to measure the index mark with the wafer in a vertical position.
It is another object of the present invention to provide a system for locating an index mark on the edge of a wafer by rotating the wafer through an angle of less than 60xc2x0.
An advantage of the system of the present invention is that the time is minimized for locating two edges of an index mark and for computing an index mark center position.
Another advantage of the system of the invention is that the center location of the index mark as well as other locations within the index mark can be readily measured upon tilting the wafer into a vertical position.
A still further advantage of the method of the present invention is that locating an index mark on the edge of a wafer by rotating the wafer through a limited rotational angle obviates the requirement for edge correction due to eccentricity between a wafer center and a center of a chuck on which the wafer is supported.
Briefly described, in a system for locating and measuring an index mark on an edge of a wafer, in accordance with the present invention, a wafer to be examined is positioned and centered on a chuck in a horizontal chuck position such that an index mark is oriented within a first and second index mark orientation feature disposed on the chuck. The chuck is translatable, tiltable between horizontal and vertical positions, and rotatable over an angle subtended by the first and second index mark orientation features of the chuck. A laser beam from a laser beam optical profiler is focussed on the surface of the wafer at a distance proximate the wafer edge and proximate the first index mark orientation feature of the chuck.
The chuck is rotated in a direction from the first to the second orientation feature through an angle of less than 60xc2x0 whereby a reflected portion of the laser beam generates data corresponding to the angular locations of index mark edges. An index mark center location is computed. The chuck is then tilted to a vertical chuck position, and the index mark is measured by the focussed laser beam to provide index mark data which serve as a basis for accepting or rejecting a wafer.
The foregoing and other features, objects and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from a reading of the following detailed description in connection with the accompanying drawings.